This invention relates to an apparatus and associated methods for dispensing fluids at a known, measurable rate. More specifically, the present invention pertains to a pump for the intravenous infusion of a medical treatment fluid in which the delivery rate is determined without direct measurement of the fluid flow rate, by using sensed pressure values and the principle of conservation of mass.
Drug delivery devices of varying construction are used to infuse medications or other biologically active substances into human or animal subjects. As used herein, the term “biologically active substance” means all types of medical and biological fluid used in the treatment of humans and animals including but not limited to peptides (such as insulin), analgesics, antiarrhythmics, steroids, hormones, nicotine, vitamins, anti-migraine medicine, anti-coagulants, local anesthetics, vaccines, allergens, muscle relaxants, etc. It should also be recognized that the apparatus is suited for the delivery of fluid into mammals, plants, fish, reptiles, and birds. The dosage levels are typically small and must be maintained over long periods of time in order to sustain a desired effect or result in the subject. A typical application is the administration of pharmaceutical preparations, where the treatment is vital for correct biological activity. The dosage delivery in such instances is often critical, and effective feedback in the form of measured flow rates is seldom available with currently used devices.
A wide variety of approaches have been offered by the prior art to meet the need for a portable device to automatically administer a substance into the vein of a subject. The need for such devices has thus been demonstrated, although the success to date has been limited. This limited success is, at least in part, due to the inability of these devices to provide a simple means to accurately determine the rate at which substance is actually delivered. Prior art devices employ such techniques as mechanical pumps, pressurized gases, pressurized liquids, or gas generating mechanisms. Because the administered substance must be maintained in a sterile environment, a flexible diaphragm is usually employed to separate it from the pump or driving gas.
The requirement for a sterile environment usually precludes direct measurement of either the rate of infusion or the total amount of the medication actually delivered. Instead, the rate of infusion is usually estimated based on the predicted rate of some other event. For instance, some devices use a pressure source in conjunction with a restrictor to regulate the flow of a driving gas. The assumed flow rate can, however, be inaccurate due to variability in pressure, temperature, viscosity of the drug being administered, or other environmental and mechanical factors. Devices using gas-generating mechanisms assume that the flow rate can be determined by correlation to an input such as a voltage. In some cases, other parameters such as body temperature or pulse rate may be used as a feedback mechanism to vary some driving force such as pressure in the cited example. In many instances, however, the precise rate at which a substance must be administered is predetermined without the availability of a feedback mechanism.
U.S. Pat. No. 4,443,218 to DeCant, Jr. et al. discloses an implantable infusate pump that utilizes a displacement reservoir to pump an infusate chamber. The displacement reservoir contains a highly viscous fluid that is provided by a second pressure reservoir that utilizes a mechanical pump. By monitoring the pressure differential across a flow restrictor located between the pressure and displacement reservoirs, the flow from the infusate chamber is indirectly measured. While this approach offers a means of measuring the flow of an administered substance, it has several disadvantages in terms of power consumption, overall life, and failsafe operation. Potential leakage of the liquid driver and the use of a mechanical pump are not conducive to sterile applications. The complex arrangement of a mechanical pump with springs and valves to control the pressure inside the pressure reservoir also makes this device more likely to fail as a result of wear.
U.S. Pat. No. 5,527,288 to Gross et al. discloses a drug delivery system based on the use of a gas-generating device. An electronic circuit is used to control the time and rate of gas generation into a chamber containing a flexible membrane. Pressure inside the chamber is used to pump a drug contained in a second chamber via the membrane common to both chambers. The generation rate is, however, dependent on environmental factors such as temperature and atmospheric pressure. The lack of a measuring means in this device thus prevents precise determination of the actual drug delivery rate.
A more complex means of controlling the drug delivery rate is also disclosed by Gross et al. in U.S. Pat. No. 6,186,982. In this patent, the same basic gas-generating means is used as in U.S. Pat. No. 5,527,288; however, a blocking member is incorporated to compensate for errors induced by temperature and pressure variations. Because of the more complex methods incorporated in this device, it is more prone to malfunction or loss of calibration than that disclosed in U.S. Pat. No. 5,527,288. Errors produced by these or other sources thus preclude an accurate determination of the drug delivery rate using this device.
U.S. Pat. No. 5,421,208 to Packard et al. discloses a device for measuring liquid parameters that is directed to peritoneal dialysis, in which a sterile aqueous solution must flow both into and out of the device. Two gas chambers are used to both control and measure the flow of a liquid contained in yet a third chamber. The first of the two gas chambers acts as either a pressure or vacuum source for the second, depending upon the direction of flow required. The second gas chamber acts as a pump for the liquid chamber via a flexible diaphragm dividing the two. The volume of the pump and liquid chamber remains constant during operation. Thus, by determining the volume of the pump chamber, the volume of the liquid chamber is determined. By monitoring the flow of gas from the source chamber and the pressure in the pump chamber, the volume of the pump chamber and, therefore, the liquid chamber can be deduced. One disadvantage of this approach is the need for a flow measurement sensor, which is more complex and prone to failure than sensors used in the art for measuring pressure. There is also a greater possibility for error in the liquid flow measurement due to loss of calibration or malfunction of the gas flow sensor.